STRESS CORROSION CRACKING EXPLICITLY CONSIDERED AS A WEATHERING PROCESS

Water is often cited as a critical factor in the mechanical weathering of rock, but the exact mechanism(s) by which water induces physical rock breakdown is poorly understood. Stress corrosion is a moisture-driven crack-propagation process that proceeds when applied stress causes atomic bonds at crack tips to become weaker. As a result, crack tips are uniquely susceptible to certain elemental exchanges between pore waters and the deforming solid, and these exchanges in turn cause cracks to widen. Although extensively documented in fracture mechanics and geophysical literature as a dominant agent of subcritical crack growth in the upper crust, stress corrosion cracking has never been explicitly considered as a rock weathering process. Here we first demonstrate that existing studies have inadvertently established that most common rock types can crack via stress corrosion under a wide range of atmospheric environmental conditions typical of Earth’s surface. Thus we hypothesize that numerous physical processes such as unloading, freezing and diurnal temperature cycling likely produce sufficient stresses in surface rocks for stress corrosion to proceed.

Considering the simple case of solar-induced thermal stresses in granite, we calculate time- and depth-dependent stress-corrosion-induced crack evolution under conditions where: 1) the rock surface is exposed to environmental water – in the form of humidity or standing water, and 2) where the surface remains dry. In addition, a simple analytical model of solar-driven fatigue crack growth, evolving under either wet or dry conditions, is described. Our preliminary results indicate that stress corrosion is an efficacious process in the context of stress associated with diurnal thermal cycling. We propose that stress corrosion is likely an extant, and possibly dominant, mechanical weathering process in most if not all Earth environments. Because the magnitude of external stresses required for rock breakdown by stress corrosion are significantly less than the fracture toughness of any given rock, the implication of our work is that ubiquitous, relatively low-stress-inducing processes such as diurnal thermal cycling potentially play a much larger role in the mechanical breakdown of rock at the Earth’s surface than previously thought.